I had the same problem. I finally traced it to where I had the external hard drive power cord plugged in. I switched it to a different surge box and it shows up every time now. Maybe this will work for you.

Can you tell us more about the drive in question and how it is connected. There are two things that occur to me:
1. If the drive is connected to a hub make sure that it is a self-powered hub (plugs into an electrical outlet). Some hard drives seem to not like being connected to only the USB bus.
2. Is this one of the drives that is USB and connects to two USB ports. Several members have complained about similar behavior with that setup. It appears that the drive draws more power than the ports can provide.

You're welcome. Glad you got that sorted out. The problem might return at some point if that hub is not "self-powered". That is one of the things that sold me on self-powered hubs. The hassle of the extra power cord is worth avoiding a recurring problem.